The present invention relates to evanescent field optical waveguide devices and methods for the manufacture of such devices.
A class of optical devices is known in which an optical channel waveguide is formed in a planar substrate comprising a core layer sandwiched between a lower cladding layer and an upper cladding layer. The channel waveguide is defined by a higher refractive index channel in the core layer. A window is formed in the substrate surface such that it overlies part of the waveguide, and has a depth through the upper cladding layer down to, and possibly into, the core layer. A sample of material can be placed in the window, and part of the evanescent field of light propagating in the waveguide will extend out of the waveguide and into the material in the window, where the light and the material interact to modify the optical field.
This can be utilized in a number of ways. The modification of the optical field can be used for frequency or amplitude modulation of the light. Alternatively, changes in the light can be used to infer properties of the material. Hence, these devices are generally modulators or detectors.
Often, a Bragg grating is included in the waveguide under the window. This provides a mechanism for determining the refractive index of the material via a spectral measurement. The presence of the sample affects the effective modal index experienced by light propagating in the grating and hence modifies the wavelength filtering response of the Bragg grating (it shifts the Bragg wavelength). The wavelength shift in light transmitted or reflected by the grating that is produced by the sample can be measured, and the refractive index of the material calculated from the size and direction of the shift.
Examples of sensors of this type can be found in WO 2006/008447 [1] and WO 2006/008448 [2].
A particularly useful technique for forming the channel waveguide is that of direct ultraviolet (UV) writing, as described in WO 2004/049024 [3]. In this technique, the core layer of the substrate is photosensitive to ultraviolet light. A spot of ultraviolet light is formed having a width the same as the desired width of the waveguide, the spot is positioned in the core layer, and the spot and the substrate are moved relative to each other to trace out the path of the waveguide. The ultraviolet light causes an increase in refractive index of the photosensitive material, thereby defining the waveguide. The spot may have a periodic intensity pattern of high and low intensity fringes, produced for example by intersecting two beams of light at an angle or by exposure through a phase mask. If the movement of the light spot relative to the substrate is at a constant velocity and exposure of the substrate to the spot is continuous, a uniform change in index is produced, giving a conventional waveguide. If exposure is discontinuous, a Bragg grating can be produced. Thus, the method provides a simple technique for creating waveguides having gratings therein in a single fabrication step.
Once the waveguide (including gratings if desired) has been written in the substrate, the sample window for the optical device can be formed. Conventionally, this is done by etching with hydrofluoric acid, which can remove the cladding and core layers to a desired depth [1, 2, 4-6]. Use of hydrofluoric acid is undesirable from safety, environmental and industrial points of view. Also, it is necessary that the etching be carried out after the waveguide has been written. Hence any error in the etching stage will ruin the otherwise completed substrate and waste the effort expended in writing the waveguide. Etching is also slow and relatively costly, and the versatility of the device is limited by the fact that the window must be positioned on top of the waveguide, so that only a limited range of configurations is possible. Also, the etched window has sharply defined edges across the waveguide which present abrupt changes to the propagating light, giving rise to undesirable back reflections and cavity effects.